An asynchronous channel for reading, writing, and manipulating a file.

An asynchronous file channel is created when a file is opened by invoking
one of the open methods defined by this class. The file contains
a variable-length sequence of bytes that can be read and written and whose
current size can be queried. The size of the file increases
when bytes are written beyond its current size; the size of the file decreases
when it is truncated.

An asynchronous file channel does not have a current position
within the file. Instead, the file position is specified to each read and
write method that initiates asynchronous operations. A CompletionHandler
is specified as a parameter and is invoked to consume the result of the I/O
operation. This class also defines read and write methods that initiate
asynchronous operations, returning a Future to represent the pending
result of the operation. The Future may be used to check if the
operation has completed, wait for its completion, and retrieve the result.

In addition to read and write operations, this class defines the
following operations:

Updates made to a file may be forced
out to the underlying storage device, ensuring that data are not
lost in the event of a system crash.

An AsynchronousFileChannel is associated with a thread pool to
which tasks are submitted to handle I/O events and dispatch to completion
handlers that consume the results of I/O operations on the channel. The
completion handler for an I/O operation initiated on a channel is guaranteed
to be invoked by one of the threads in the thread pool (This ensures that the
completion handler is run by a thread with the expected identity).
Where an I/O operation completes immediately, and the initiating thread is
itself a thread in the thread pool, then the completion handler may be invoked
directly by the initiating thread. When an AsynchronousFileChannel is
created without specifying a thread pool then the channel is associated with
a system-dependent default thread pool that may be shared with other
channels. The default thread pool is configured by the system properties
defined by the AsynchronousChannelGroup class.

Channels of this type are safe for use by multiple concurrent threads. The
close method may be invoked at any time, as specified
by the Channel interface. This causes all outstanding asynchronous
operations on the channel to complete with the exception AsynchronousCloseException. Multiple read and write operations may be
outstanding at the same time. When multiple read and write operations are
outstanding then the ordering of the I/O operations, and the order that the
completion handlers are invoked, is not specified; they are not, in particular,
guaranteed to execute in the order that the operations were initiated. The
ByteBuffers used when reading or writing are not
safe for use by multiple concurrent I/O operations. Furthermore, after an I/O
operation is initiated then care should be taken to ensure that the buffer is
not accessed until after the operation has completed.

As with FileChannel, the view of a file provided by an instance of
this class is guaranteed to be consistent with other views of the same file
provided by other instances in the same program. The view provided by an
instance of this class may or may not, however, be consistent with the views
seen by other concurrently-running programs due to caching performed by the
underlying operating system and delays induced by network-filesystem protocols.
This is true regardless of the language in which these other programs are
written, and whether they are running on the same machine or on some other
machine. The exact nature of any such inconsistencies are system-dependent
and are therefore unspecified.

Method Detail

open

Opens or creates a file for reading and/or writing, returning an
asynchronous file channel to access the file.

The options parameter determines how the file is opened.
The READ and WRITE options determines if the file should be opened for reading and/or
writing. If neither option is contained in the array then an existing file
is opened for reading.

If this option is present then a new file is created, failing if
the file already exists. When creating a file the check for the
existence of the file and the creation of the file if it does not exist
is atomic with respect to other file system operations. This option is
ignored when the file is opened only for reading.

If this option is present then an existing file is opened if it
exists, otherwise a new file is created. When creating a file the check
for the existence of the file and the creation of the file if it does
not exist is atomic with respect to other file system operations. This
option is ignored if the CREATE_NEW option is also present or
the file is opened only for reading.

When this option is present then the implementation makes a
best effort attempt to delete the file when closed by the
the close method. If the close method is not
invoked then a best effort attempt is made to delete the file
when the Java virtual machine terminates.

The executor parameter is the ExecutorService to
which tasks are submitted to handle I/O events and dispatch completion
results for operations initiated on resulting channel.
The nature of these tasks is highly implementation specific and so care
should be taken when configuring the Executor. Minimally it
should support an unbounded work queue and should not run tasks on the
caller thread of the execute method.
Shutting down the executor service while the channel is open results in
unspecified behavior.

The attrs parameter is an optional array of file file-attributes to set atomically when creating the file.

The new channel is created by invoking the newFileChannel method on the
provider that created the Path.

Parameters:

file - The path of the file to open or create

options - Options specifying how the file is opened

executor - The thread pool or null to associate the channel with
the default thread pool

attrs - An optional list of file attributes to set atomically when
creating the file

UnsupportedOperationException - If the file is associated with a provider that does not
support creating asynchronous file channels, or an unsupported
open option is specified, or the array contains an attribute that
cannot be set atomically when creating the file

The resulting channel is associated with default thread pool to which
tasks are submitted to handle I/O events and dispatch to completion
handlers that consume the result of asynchronous operations performed on
the resulting channel.

truncate

If the given size is less than the file's current size then the file
is truncated, discarding any bytes beyond the new end of the file. If
the given size is greater than or equal to the file's current size then
the file is not modified.

force

Forces any updates to this channel's file to be written to the storage
device that contains it.

If this channel's file resides on a local storage device then when
this method returns it is guaranteed that all changes made to the file
since this channel was created, or since this method was last invoked,
will have been written to that device. This is useful for ensuring that
critical information is not lost in the event of a system crash.

If the file does not reside on a local device then no such guarantee
is made.

The metaData parameter can be used to limit the number of
I/O operations that this method is required to perform. Passing
false for this parameter indicates that only updates to the
file's content need be written to storage; passing true
indicates that updates to both the file's content and metadata must be
written, which generally requires at least one more I/O operation.
Whether this parameter actually has any effect is dependent upon the
underlying operating system and is therefore unspecified.

Invoking this method may cause an I/O operation to occur even if the
channel was only opened for reading. Some operating systems, for
example, maintain a last-access time as part of a file's metadata, and
this time is updated whenever the file is read. Whether or not this is
actually done is system-dependent and is therefore unspecified.

This method is only guaranteed to force changes that were made to
this channel's file via the methods defined in this class.

Parameters:

metaData - If true then this method is required to force changes
to both the file's content and metadata to be written to
storage; otherwise, it need only force content changes to be
written

lock

This method initiates an operation to acquire a lock on the given
region of this channel's file. The handler parameter is a
completion handler that is invoked when the lock is acquired (or the
operation fails). The result passed to the completion handler is the
resulting FileLock.

The region specified by the position and size
parameters need not be contained within, or even overlap, the actual
underlying file. Lock regions are fixed in size; if a locked region
initially contains the end of the file and the file grows beyond the
region then the new portion of the file will not be covered by the lock.
If a file is expected to grow in size and a lock on the entire file is
required then a region starting at zero, and no smaller than the
expected maximum size of the file, should be locked. The two-argument
lock(Object,CompletionHandler) method simply locks a region
of size Long.MAX_VALUE. If a lock that overlaps the requested
region is already held by this Java virtual machine, or this method has
been invoked to lock an overlapping region and that operation has not
completed, then this method throws OverlappingFileLockException.

Some operating systems do not support a mechanism to acquire a file
lock in an asynchronous manner. Consequently an implementation may
acquire the file lock in a background thread or from a task executed by
a thread in the associated thread pool. If there are many lock operations
outstanding then it may consume threads in the Java virtual machine for
indefinite periods.

Some operating systems do not support shared locks, in which case a
request for a shared lock is automatically converted into a request for
an exclusive lock. Whether the newly-acquired lock is shared or
exclusive may be tested by invoking the resulting lock object's isShared method.

File locks are held on behalf of the entire Java virtual machine.
They are not suitable for controlling access to a file by multiple
threads within the same virtual machine.

Parameters:

position - The position at which the locked region is to start; must be
non-negative

size - The size of the locked region; must be non-negative, and the sum
position + size must be non-negative

shared - true to request a shared lock, in which case this
channel must be open for reading (and possibly writing);
false to request an exclusive lock, in which case this
channel must be open for writing (and possibly reading)

attachment - The object to attach to the I/O operation; can be null

handler - The handler for consuming the result

Throws:

OverlappingFileLockException - If a lock that overlaps the requested region is already held by
this Java virtual machine, or there is already a pending attempt
to lock an overlapping region

lock

This method initiates an operation to acquire a lock on the given
region of this channel's file. The handler parameter is a
completion handler that is invoked when the lock is acquired (or the
operation fails). The result passed to the completion handler is the
resulting FileLock.

An invocation of this method of the form ch.lock(att,handler)
behaves in exactly the same way as the invocation

lock

This method initiates an operation to acquire a lock on the given
region of this channel's file. The method behaves in exactly the same
manner as the lock(long, long, boolean, Object, CompletionHandler)
method except that instead of specifying a completion handler, this
method returns a Future representing the pending result. The
Future's get method returns the FileLock on successful completion.

Parameters:

position - The position at which the locked region is to start; must be
non-negative

size - The size of the locked region; must be non-negative, and the sum
position + size must be non-negative

shared - true to request a shared lock, in which case this
channel must be open for reading (and possibly writing);
false to request an exclusive lock, in which case this
channel must be open for writing (and possibly reading)

Returns:

a Future object representing the pending result

Throws:

OverlappingFileLockException - If a lock is already held by this Java virtual machine, or there
is already a pending attempt to lock a region

lock

This method initiates an operation to acquire an exclusive lock on this
channel's file. The method returns a Future representing the
pending result of the operation. The Future's get method returns the FileLock on successful completion.

An invocation of this method behaves in exactly the same way as the
invocation

tryLock

Attempts to acquire a lock on the given region of this channel's file.

This method does not block. An invocation always returns immediately,
either having acquired a lock on the requested region or having failed to
do so. If it fails to acquire a lock because an overlapping lock is held
by another program then it returns null. If it fails to acquire
a lock for any other reason then an appropriate exception is thrown.

Parameters:

position - The position at which the locked region is to start; must be
non-negative

size - The size of the locked region; must be non-negative, and the sum
position + size must be non-negative

OverlappingFileLockException - If a lock that overlaps the requested region is already held by
this Java virtual machine, or if another thread is already
blocked in this method and is attempting to lock an overlapping
region of the same file

OverlappingFileLockException - If a lock that overlaps the requested region is already held by
this Java virtual machine, or if another thread is already
blocked in this method and is attempting to lock an overlapping
region

read

Reads a sequence of bytes from this channel into the given buffer,
starting at the given file position.

This method initiates the reading of a sequence of bytes from this
channel into the given buffer, starting at the given file position. The
result of the read is the number of bytes read or -1 if the given
position is greater than or equal to the file's size at the time that the
read is attempted.

read

Reads a sequence of bytes from this channel into the given buffer,
starting at the given file position.

This method initiates the reading of a sequence of bytes from this
channel into the given buffer, starting at the given file position. This
method returns a Future representing the pending result of the
operation. The Future's get method returns
the number of bytes read or -1 if the given position is greater
than or equal to the file's size at the time that the read is attempted.

This method works in the same manner as the AsynchronousByteChannel.read(ByteBuffer) method, except that bytes are
read starting at the given file position. If the given file position is
greater than the file's size at the time that the read is attempted then
no bytes are read.

Parameters:

dst - The buffer into which bytes are to be transferred

position - The file position at which the transfer is to begin;
must be non-negative

write

Writes a sequence of bytes to this channel from the given buffer, starting
at the given file position.

This method works in the same manner as the AsynchronousByteChannel.write(ByteBuffer,Object,CompletionHandler)
method, except that bytes are written starting at the given file position.
If the given position is greater than the file's size, at the time that
the write is attempted, then the file will be grown to accommodate the new
bytes; the values of any bytes between the previous end-of-file and the
newly-written bytes are unspecified.

Parameters:

src - The buffer from which bytes are to be transferred

position - The file position at which the transfer is to begin;
must be non-negative

write

Writes a sequence of bytes to this channel from the given buffer, starting
at the given file position.

This method initiates the writing of a sequence of bytes to this
channel from the given buffer, starting at the given file position. The
method returns a Future representing the pending result of the
write operation. The Future's get method
returns the number of bytes written.

This method works in the same manner as the AsynchronousByteChannel.write(ByteBuffer) method, except that bytes are
written starting at the given file position. If the given position is
greater than the file's size, at the time that the write is attempted,
then the file will be grown to accommodate the new bytes; the values of
any bytes between the previous end-of-file and the newly-written bytes
are unspecified.

Parameters:

src - The buffer from which bytes are to be transferred

position - The file position at which the transfer is to begin;
must be non-negative